What does the word 'Hallelujah' mean?

Hallelujah comes from two Hebrew words 'Halelu' which means everybody praise and 'Yah' which comes from the word for God. So it means everybody praise God.

 
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In Old Testament times the word 'Hallelujah' was used to tell people to praise God, now it is a word that Christians say when they are praising God.